Procedures employing HCHO denaturation followed by Poly A Sepharose chromatography have permitted the separation of HeLa cell mRNA into four classes: 1) Poly A (plus) Oligo U (minus) (50%); 2) Poly A (plus) Oligo U (plus) (15%); 3) Poly A (minus) Oligo U (plus) (5%); 4) Poly A (minus) Oligo U (minus) (30%). These four classes of RNA are similar in size (heterogeneous) and metabolic stability and are found in both free and membrane bound polysomes. The drug DRB has been found to selectively inhibit the production of cytoplasmic Poly A (plus) mRNA and ribosomal RNA while having little effect on 5S and transfer RNA.